The majority of persons seeking treatment for drug dependence are polydrug abusers. Marijuana is the drug used most frequently by other illicit drug abusers and alcoholics. For example, 25-70% of person's seeking treatment for cocaine and alcohol dependence also use marijuana, and many of these individuals are reluctant to discontinue its use. While such marijuana use is easily documented, there is a dearth of scientific information concerning its influence on treatment outcome in drug-dependent individuals. This absence of information prevents clinicians from making empirically-based decisions regarding how to treat marijuana use in these populations. The research proposed in this application will begin to address this problem by conducting an experimental analysis of the effects of behavioral interventions targeting marijuana use during treatment for two types of substance dependence, i.e., cocaine and alcohol dependence. The proposed studies will (1) assess how marijuana use affects treatments for cocaine and alcohol dependence, and (2) develop and assess theoretically-based interventions for effectively treating marijuana abuse in these clinical populations. Study 01 will assess the efficacy of a behavioral intervention targeting marijuana abstinence during treatment of cocaine dependence. A contingency-management procedure will be employed to engender short-term marijuana abstinence during treatment. Study 02 will systematically replicate the procedures used in Study 01, and will assess the efficacy of combining contingency-management procedures and motivational interviewing to engender longer-term marijuana abstinence. If motivational interviewing does not result in greater duration of marijuana abstinence, then Study 03A will be conducted to examine the efficacy of combining an additional contingency- management procedure with that used in Study 01 to engender longer-term marijuana abstinence. This additional procedure will involve providing reinforcement contingent on participation in prosocial activities that are incompatible with or substitute for marijuana use. If in Study 02 motivational interviewing effectively engenders longer-term marijuana abstinence, then Study 03B will be conducted to extend these efforts by targeting marijuana abstinence during outpatient treatment for alcohol dependence. Results from these studies will provide information on whether the experimental interventions can engender marijuana abstinence during and following treatment for cocaine and alcohol dependence, and how these interventions affect other treatment outcome variables such as treatment retention, cocaine and alcohol abstinence, psychosocial functioning, and prosocial behavior. Overall, this research will (1) contribute to the development of theoretically consistent treatments for drug dependence, (2) produce new scientific information on the influence of marijuana use on outcome in patients receiving outpatient behavioral treatment for cocaine and alcohol dependence, and (3) provide important information that can be used by clinicians to provide effective treatments for polydrug abuse.